Electrical safety standards have been established to assure that basic precautionary steps are taken to prevent the more readily avoidable mishaps, and while more sophisticated fault detection devices can offer special protection, most electrical devices must rely upon adherence to basic, established standards and practices to reduce the possibilities of shock and fire. Unfortunately, many of the basic protections are nullified by simple connective errors in receptacle outlets or in appliance installations. Plug and socket design can provide a positive assurance that plugs are inserted correctly and cord set wiring is most frequently well controlled as a matter of standardized manufacture, but wiring of receptacle outlets or of installed utilization equipment is another matter and generally subject to many individual circumstances.
This invention relates to the protection of electrical utilization equipment which has critical safety requirements in its connection to a current source, wherein reversal of ungrounded and grounded neutral conductors or the absence of a connection to a grounding conductor for grounding of exposed noncurrent carrying conductive parts of the equipment would constitute a hazardous condition.
While plug-in test devices are available that connect indicator lamps across the outlet terminals to provide information of wiring errors, the extent of use and response to their indications is discretionary and such devices have no positive protective function.
It is well understood that providing this invention may be cost prohibitive, in many instances. It can however, be very inexpensively incorporated into the design of various power control devices which already employ a relay for load current switching, such as many of the faul interrupters and other devices which turn current on and off in accordance with such factors as changes in temperature, light intensity or the reception of radio or infra-red signals. It may actually save money in many instances by eliminating extra relay contacts where it has been considered necessary to interrupt both the grounded and the ungrounded conductor in order to avoid the hazardous contingency of their possible reversal.
A more expensive relay operated system that provides connective protection with interruption of both the grounded and ungrounded conductors, is disclosed in an embodiment of the "Universal Fault Circuit Interrupter", as it appears in U.S. Pat. No. 4,707,759, dated Nov. 17, 1987. The patented system employs a set of normally closed contacts that connect the ground side of the relay coil to the grounding conductor. This serves to supply only an initial energizing current with the ground side of the coil being reconnected to the grounded conductor upon closure of the relay as a set of normally open contacts close simultaneously with two other sets of normally open contacts that act to connect the ungrounded and the grounded conductor to the load. This prior patented system will not be energized if a grounding connection is initially absent but will not be deenergized if the grounding connection is interrupted during use, as it will be with the present invention. A direct connection of the relay coil to the grounding conductor, which would permit continuous monitoring of the grounding connection, was not provided in the patented system, since this would pass a small but continuing line current into the grounding conductor, which is not to be employed as a current carrying conductor, with regard to the supplying of current from the source to utilization equipment.
In accordance with the present invention, only a single pole, single throw relay is used to open and close the connection of the ungrounded conductor to the load. In most prototype embodiments, heretofore made, the relay has been a very compact and inexpensive d.c. coil power type, designed for printed circuit use, and soon to be produced with an a.c. coil, having contacts capable of switching 30 amperes at 240 volts. This and other small relays with even higher current carrying capacities may be obtained at relatively low cost, which further enhances the usefulness of the invention. Other components, exclusive of hardware, are a pair of rectifier diodes and an optional control which may be a small manually operated switch, or another switching element such as a thyristor.